SEM Rust-seal and welding Options

[Wes V]

I'm addressing rust issues and came upon an problem that I'm unsure about.

The top surface of the longtudinal, under the (removed) battery tray, had a lot of surface rust but is solid.

After light sanding and wire brushing I treated the surface with SEM Rust-seal (39308). This is a rust "converter" that turns the rust into a permanent black coating. The product works great and as advertised, however I think it leads to the problem I'm having.

I want to weld an additional coverplate over this area and fabricated one using 18 gauge plate steel and drilled 1/4" holes in it for welding. A light coat of 3M weld-through was sprayed on both the long and metal plate.

When I try to mig weld up the holes, attaching the plate to the long, I get a great looking weld that doesn't attach to the metal of the "long" at all!!!

I even tried scratching off any form of paint (in the holes) with a dental pick.

So, two things come to mind.

It may be that the SEM is preventing the weld.

It could also be that 1/4" holes are too small.

Most of the SEM coating can be removed with Aircraft paint stripper (dang, what a messy job)

Wes Vann